A Saudi vessel that had been due to load weapons at a northern French port on Friday set sail without them and headed for Spain, a day after a rights group tried to block the cargo on humanitarian grounds.

A French judge threw out that legal challenge but the Bahri-Yanbu set course for Santander shortly after minus the weapons, officials said and ship-tracking data showed.

The saga is an embarrassment for President Emmanuel Macron, who on Thursday defended arms sales to Saudi Arabia.

Riyadh leads the pro-government military coalition in the four-year civil war that has devastated Yemen, killed tens of thousands and left much of the population on the brink of famine.

Macron said on Thursday Riyadh, which he called a key ally in the fight against terrorism, had assured him the weapons the ship was to load were not to be used against civilians.

An official working for Jean-Paul Lecoq, the opposition Communist Member of Parliament for port city Le Havre, confirmed the vessel had left without the consignment.

"This is a lesson for the executive," he told Reuters. "It can no longer give bland statements saying 'do not worry, we have guarantees'. That no longer works."

European powers are split over arms sales to Saudi Arabia, with France and Britain lobbying against German efforts to toughen the way they are regulated.

The Bahri-Yanbu had been at anchor 25 kilometres (15 miles) off Le Havre since Wednesday evening, already carrying a separate consignment of arms loaded in Antwerp.

War Crimes Allegations

France's defence ministry referred questions about the consignment to the foreign ministry, which referred Reuters back to the defence ministry. Neither the prime minister's office, which approves arms' sales, nor the presidency responded.

A Saudi embassy spokesman could not immediately comment.

The move by ACAT came after online investigative site Disclose published leaked military intelligence showing weapons sold by France to Saudi Arabia, including tanks and laser-guided missile systems, were being used against civilians in Yemen.

ACAT had argued that the transfer contravened the U.N. Arms Trade Treaty, which says one country cannot authorise the transfer of weapons if it knows at the time that those weapons could be used to commit war crimes or target civilians.

U.N. officials have said all sides in the Yemeni conflict may have committed war crimes.

The government declined to give details of the arms order, which Disclose had said included eight Caesar howitzer cannons.